Light-intensifying device

ABSTRACT

A light-intensifying device comprises a panel of integrated layers composed of a transparent base, a first transparent electrode layer, a photoconductive layer, a negative resistance layer, an electroluminescent layer, and a second transparent electrode. The device can prevent the halation appearing around the bright field of an output image.

United States Patent 1191 Hayakawa Feb. 5, 1974 LIGHT-INTENSIFYING DEVICE 3,564,260 2/1971 Tanoka... 250/213 3,711,719 1/1973 Sze e5i.... 250/213 R [75] Inventor: Hayakawa, Osaka Japan 3,740,616 6/1973 Suzfiki 317/234 R [73] Assignee: Matsushita Electric Industral Co., 2

, eeg Osaka Japan 3,590,253 6/1971 Novice 250/213 [22] Filed: Feb. 16, 1973 3,699,374 10 1972 SChO" 313 94 [21] Appl' No; 332793 Primary Examiner-Martin H. Edlow Attorney, Agent, or Firm-E. F. Wenderoth et a1. [30] Foreign Application Priority Data Feb. 22, 1972 Japan 47-18691 52 US. en 317/234 R, 317/235 N, 250/213, A light-intensifying device Comprises a Panel of inte- 313 1 3 grated layers composed of a transparent base, a first 51 1111.61. H01! 15/00 transparent electrode layer, a photoconductive layer, 58 Field 61 Search 250/213; 317/235 N a negative resistance layer, an electroluminescent layer, and a second transparent electrode. The device 5 References Cited can prevent the halation appearing around the bright UNITED STATES PATENTS field 0f 3,548,214 12/1970 Brown 250/213 5 Claims, 4 Drawing Figures TRA NSPARENT ELECTRODE LIGHT-TO-ELECTRICITYCONVERSION NEGATIVE RESISTANCE ELECTRICITY-TO-LIGHT CONVERSION TRANSPARENT ELECTRODE GLASS PATENIED 51914 $790,867

SHEET 1 0F 2 6 C W TRANSPARENT ETECTROOE 5 LlGHT-TO-ELECTRICITYCONVERSION 4 NEGATIVE RESISTANCE 3 ELECTRICITY-TO-LIGHT CONVERSION f TRANSPARENT ELECTRODE OLAss FIG/l CURRENT VOLTAGE PMENTEDFEB 3.790%? sum 2' a? 2 CURRENT I VOLTAGE FIGJ I LlGHT-INTENSIFYING DEVICE This invention relates to a light-intensifying device, and particularly to a novel and improved lightintensifying device which can prevent halation appearing around the bright field of an output image when increasing the contrast of the image in a dark field.

A conventional light-intensifying device has layers, an electroluminescent layer and a photoconductive layer, which are sandwiched between two transparent electrodes, and the transparent electrodes are usually made from thin layers of tin oxide deposited on transparent glass plates. Usually, the electroluminescent layer is composed of zinc sulfide or zinc selenide as the active material, and the photoconductive layer is composed of cadmium sulfide or cadmium selenide as the active material. The input light is projected on the transparent electrodes adjacent to the photoconductive layer, and then an output light is observed at the other transparent electrode adjacent to the electroluminescent layer. Such a conventional device operates on the following principle: When a voltage is applied across the two transparent electrodes and input light is projected on the photoconductive layer, the voltage across the photoconductive layer decreases in accordance with the decrease of the resistivity of the photoconductive layer upon projection of input light thereon. Accordingly, the voltage across the electroluminescent layer increases, and so the input light is intensified. When the input light is composed of a light image, an intensified light image corresponding to the input light image can be observed.

Such a conventional light-intensifying device has a drawback that it is rather difficult to enchance the contrast of the dark field in the output light image. For example, an increase of the intensity of the input light causes halation, i.e., a halo, around the bright field in the output image.

Therefore, a fundamental object of this invention is to provide a novel and improved light-intensifying device which can prevent the halation appearing around the bright field in the output light image.

A further object of this invention is to provide a lightintensifying device including a negative resistance layer in addition to an electroluminescent layer and a photoconductive layer.

These objects are achieved by providing a lightintensifying device which comprises a panel of integrated layers consisting essentially of a transparent base; a first transparent electrode layer; an electricityto-light conversion layer having a specific electric resistance, R; a negative resistance layer having a voltagecurrent curve which has a transition point from a low resistance branch having a specific resistance Rl to a high branch having a specific resistance R,, during an increase of the voltage the transition point being at a critical voltage V a light-to-electricity conversion layer having specific electric resistances R, and R when irradiated with light having intensities L and L respectively, where L, is lower than and L is higher than a critical intensity L,., which causes said negative resistance layer to receive a voltage equal to said critical voltage V, when said light-intensifying device is provided with a given voltage V; and a second transparent electrode layer, said first transparent electrode layer, said electricity-toJight conversion layer, said negative resistance layer, said light-to-electricity conversion layer, and said second transparent electrode layer being stacked in the recitedl order, whereby upon irradiation of a light image on said device to which a voltage V is applied, a unit area of said electricity-tolight conversion layer is energized by a voltage RV/(R,+R+Rl) or RV/(R -l-R+lR,,) when the corresponding unit area of said light-to-electricity conversion'layer is irradiated with light of intensity L, or L and accordingly the halation around the bright field of the light image is eliminated.

These and other objects of this invention will be apparent upon consideration of the following detailed description taken together with the accompanying drawings wherein:

FIG. I is a schematic view of the fundamental construction of a light-intensifying device according to this invention; I

FIG. 2 is a graph of the voltage versus current characteristic of a negative resistance layer of the device shown in FIG. 1;

FIG. 3 is a graph of the voltage versus current characteristics of a thermistor having a positive temperature coefiicient of resistance for explaning the operation of the device of the invention; and

FIG. 4 is a schematic view of the construction of a light-intensifying device of this invention having a heating element embedded in a negative resistance layer.

Referring to FIG. 1, a light-intensifying device according to the invention comprises a panel of integrated layers as shown in FIG. 1. A first transparent electrode layer 2 is deposited on a transparent base 1 such as a glass plate. An electricity-to-light conversion layer 3 is formed on said first transparent electrode layer 2. A negative resistance layer 4 is formed on said electricity-to-light conversion layer 3. A light-to electricity conversion layer 5 is formed on said negative resistance layer 4. A second transparent electrode layer 6 is deposited on said light-to-eljectricity conversion layer 5.

FIG. 2 shows the voltage versus current characteristic of said negative resistance layer 4. Said negative resistance layer 4 has a transition point 7 on the voltagecurrent characteristic curve at a critical voltage V At that transition point 7, the resistance changes from a low resistance branch 8 having a specific resistance Rl to a high resistance branch 9 having a specific resistance R,, with an increase in the voltage, as shown in FIG. 2. The value of R,, is higher at least by times than the value of R1. Said light-to-electricity conversion layer 5 has specific resistances R and R for irradiation of light of intensities, L and L respectively, where L, is lower than and L is higher than a critical intensity L which causes said negative resistance layer 4 to receive a voltage equal to said critical voltage V The value of R is higher than the value of R When a certain voltage V is applied to said lightintensifying device and a unit area of said light-toelectricity conversion layer 5 is irradiated with light having an intensity L or L the voltage appearing at the corresponding unit area of said electricity-to-light conversion layer 3 is expressed as RV/(R +R+Rl) or RV/(R +R+R respectively, where Ris the specific resistance of said electricity-to-light conversion layer 3.

The brightness B of the light emitted frorri said electricity-'to-lightconversion layer 3 is related to the voltage V appearing at the layer 3 and it is expressed as follows;

B KV H where n is, in practice, a value between 3 and 7, and K is a constant. Therefore, the brightness of the light emitted from a unit area of said electricity-to-light conversion layer 3 decreases in accordance with an increase of the light intensity from L, to L oflight irradiated on the corresponding unit area of said light-toelectricity conversion layer 5. Consequently, a clear light image in a dark field can be obtained without appearance of halation around the very bright field of the light image.

Said first transparent electrode layer 2 and said second transparent electrode layer 6 can be made of transparent thin films of tin oxide. Said electricity-to-light conversion layer 3 consists essentially of an electroluminescent material such as zinc sulfide or zinc selenide. Said light-to-electricity conversion layer 5 consists essentially of a photoconductive material such as cadmium sulfide or cadmium selenide. Said negative resistance layer 4 having a voltage-current characteristic curve as shown in FIG. 2 consists essentially of a thermistor element having a positive temperature coefficient of resistance (designated as a PTC thermistor in the following description). Said PTC thermistor layer is fabricated by applying a paste having powdered PTC thermistor dispersed in a vehicle to said electricity-to-light conversion layer 3 and curing said paste.

It is desirable to be able to change the critical voltage V easily so as to prevent halation from appearing around the bright field at various brightness. As shown in FIG. 3, the voltage-current curve of said PTC thermistor shifts from a curve to a curve 12 through a curve 11 in accordance with an increase of the temperature of said PTC thermistor, and consequently the critical voltage shifts also from V to V through V Thus the critical voltage V can, be shifted as desired by appropriately heating the device.

Referring to FIG. 4, in a panel comprising integrated layers consisting of a transparent base 16, a first transparent electrode layer 17, an electricity-to-light conversion layer 18, a negative resistance layer 19, a lightto-electricity conversion layer and a second transparent electrode layer 21, which are stacked in the recited order, a heating element 22 is embedded in said negative resistance layer 18. Said heating element 22 consists of a metallic wire or a metallic thin film. Said negative resistance layer 19 consists essentially of a PTC thermistor element. The temperature of said negative resistance layer 19 varies with the electric current flowing through said heating element 22, and accord-.

ingly the critical voltage, V can be controlled by adjusting the heating current.

While there have been described at present what are believed to be the preferred embodiments of this invention, it will be obvious that various changes and modifications can be made therein without departing from the spirit or scope of the invention.

What is claimed is:

l. A light-intensifying device comprising a panel of integrated layers consisting essentially of a transparent base; a first transparent electrode layer; an electricityto-light conversion layer having a specific electric resistance R; a negative resistance layer having a voltagecurrent curve which has a transition point from a low resistance branch having'a specific resistance R! to a high resistance branch having a specific resistance R with an increase of the voltage, the transition point being at a critical voltage V a light-to-electricity conversion layer having specific electric resistances R and R in respect to irradiation of light having intensities L, and L respectively, where L is lower than L is higher than a critical intensity L which causes said negative resistance layer to receive a voltage equal to said critical voltage V when said light-intensifying device has a given voltage V applied thereto; and a second transparent electrode layer, said first transparent electrode layer, said electricity-to-light conversion layer, said negative resistance layer, said light-to-electricity conversion layer, and said second transparent electrode layer being stacked in the recited order, whereby upon irradiation of a light image on said device having said given voltage V applied thereto, a unit area of said electricity-to-light conversion layer is energized by a voltage RV/(R,+R+Rl) or RV/(R +R+R,,) when the corresponding unit area of said light-to-electricity conversion layer is irradiated with light having a light intensity 1 or L and accordingly halation around the bright field of the light image is eliminated.

2. A light-intensifying device as claimed in claim 1, wherein said electricity-to-light conversion layer consists essentially of an electroluminescent material.

3. A light-intensifying device as claimed in claim 1, wherein said negative resistance layer consists essentially of a thermistor layer comprised of a finely divided powder having a positive temperature coefficient of resistance. I

4. A light-intensifying device as claimed in claim 1, wherein said light-to-electricity conversion layer consists essentially of a photoconductive material.

5. A light-intensifying device as claimed in claim 1, wherein said negative resistance layer has a heating element embedded therein for controlling the size of said critical voltage, V,. 

2. A light-intensifying device as claimed in claim 1, wherein said electricity-to-light conversion layer consists essentially of an electroluminescent material.
 3. A light-intensifying device as claimed in claim 1, wherein said negative resistance layer consists essentially of a thermistor layer comprised of a finely divided powder having a positive temperature coefficient of resistance.
 4. A light-intensifying device as claimed in claim 1, wherein said light-to-electricity conversion layer consists essentially of a photoconductive material.
 5. A light-intensifying device as claimed in claim 1, wherein said negative resistance layer has a heating element embedded therein for controlling the size of said critical voltage, Vc. 